1. Technical Field of the Invention
The present invention relates to electronic components and ladder filters, and more particularly, to an electronic component, such as a ladder filter, including a piezoelectric resonator which uses a mechanical resonance of a piezoelectric member.
2. Description of the Related Art
FIG. 15 is a perspective view of a conventional piezoelectric resonator which provides a background of the present invention described below. A piezoelectric resonator 1 includes a piezoelectric substrate 2 having, for example, a rectangular plate shape. The piezoelectric substrate 2 is polarized in the thickness direction. On both major surfaces of the piezoelectric substrate 2, external electrodes 3 are provided. When a signal is input between the external electrodes 3, an electric field is applied to the piezoelectric substrate 2 in the thickness direction and the piezoelectric substrate 2 vibrates in the longitudinal direction.
The piezoelectric resonator shown in FIG. 15 is an unstiffened type, in which the vibration direction differs from the direction of polarization and the electrical field. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator, in which the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. This causes a frequency bandwidth to be narrow when an unstiffened piezoelectric resonator is used as a filter. Therefore, the degree of freedom in characteristics design is low in electronic components using such a piezoelectric resonator, including a filter and an oscillator.
The piezoelectric resonator shown in FIG. 15 uses the first-order resonance in the longitudinal mode. Also, as a result of the structure of the piezoelectric resonator shown in FIG. 15, this resonator generates large spurious resonances in odd-number harmonic modes, such as the third-order and fifth-order modes, and in the width mode. To suppress these spurious resonances, some corrective measures have been considered, such as polishing, increasing mass, and changing the shape of the electrodes. These measures increase manufacturing cost.
In addition, since the piezoelectric substrate has a rectangular plate shape, the substrate cannot be made thinner because of minimum strength requirements. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be made large. This is extremely inconvenient for achieving impedance matching with an external circuit. To constitute a ladder filter by connecting a plurality of piezoelectric resonators in series and in parallel alternately, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation. Because a piezoelectric resonator has the shape restriction described above, however, a large attenuation cannot be obtained.
In Japanese Unexamined Patent Publication No. 8-110475, which has been filed by the same applicant as in the present application, a piezoelectric resonator having a lamination structure which generates a longitudinal vibration in a basic mode has been proposed, in which a plurality of piezoelectric layers and a plurality of electrodes are laminated to form a base member having a longitudinal direction, and the plurality of piezoelectric layers are polarized in the longitudinal direction of the base member. A piezoelectric resonator having such a lamination structure is a stiffened type and has the piezoelectric layers arranged such that the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same. The piezoelectric resonator has a smaller spurious resonance and a larger difference .DELTA.F between the resonant frequency and the antiresonant frequency than an unstiffened piezoelectric resonator.
To produce a ladder filter with the use of a piezoelectric resonator having such a lamination structure, a structure shown in FIGS. 16 to 20, for example, can be considered.
FIG. 16 is an exploded perspective view of a ladder filter which provides a background of the present invention. FIG. 17 is a plan view of the ladder filter, and FIG. 18 is an elevation view of the ladder filter. FIG. 19 is a side view of the ladder filter, and FIG. 20 is a circuit diagram of the ladder filter. In the ladder filter 4 shown in FIGS. 16 to 20, four pattern electrodes 6a, 6b, 6c, and 6d are provided on an insulating substrate 5. Piezoelectric resonators 1a, 1b, 1c and 1d having the lamination structure described above are electrically connected to these pattern electrodes 6a to 6d. Two external electrodes 3a and 3b are provided with a gap in the width direction of each of the piezoelectric resonators 1a, 1b, 1c, and 1d on one side surface of each of the piezoelectric resonators 1a to 1d. At the centers of the external electrodes 3a and 3b in the longitudinal direction, support members 7 made from an electrically conductive material are provided. These support members 7 are bonded and connected to the pattern electrodes 6a to 6d via electrically conductive adhesive 8. This ladder filter 4 has a ladder circuit shown in FIG. 20. A metal cap (not shown) is placed on the substrate to cover the piezoelectric resonators 1a to 1d.
In the ladder filter 4 shown in FIGS. 16 to 20, since the external electrodes 3a and 3b are provided on one side surface of each of the piezoelectric resonators 1a to 1d, and the support members 7 made from the electrically conductive material are provided on the external electrodes 3a and 3b, the piezoelectric resonators 1a to 1d can be surface-mounted on the insulating substrate 5.
In the ladder filter 4 shown in FIGS. 16 to 20, different electrically conductive adhesives 8 disposed at adjacent areas may contact each other, electrically conductive adhesive 8 may contact a pattern electrode which is located nearby, or electrically conductive adhesive 8 may contact the support member of an adjacent adhesive 8 as shown in FIGS. 16 to 18. This leads to a short circuit between adjacent external electrodes 3a and 3b of the piezoelectric resonators 1a to 1d. This condition becomes prominent as the distances between the pattern electrodes 6a to 6d and the distances between the external electrodes 3a and 3b are narrowed to make the ladder filter compact.